A variety of electronic devices and circuits must be supplied with a voltage having a magnitude that is precisely controlled using a voltage regulator, charge pump or other voltage source. The magnitude of the voltage supplied by the voltage regulator, charge pump or other source is often set by the magnitude of a reference voltage. Various reference voltage generators are well known in the art.
A common reference voltage generator is a diode voltage generator, a typical example of which is shown in FIG. 1. The diode voltage generator 10 includes a reference current source 14 directing a reference current IR through a bipolar PNP transistor 20 having its base and collector interconnected in a diode configuration. A PN junction diode forward voltage VEB is produced at the emitter of the transistor 20. Of course, other designs for a diode voltage generator can be used, such as one having a bipolar NPN transistor.
The current-voltage characteristics of the diode voltage generator 10 are shown in FIG. 2. As shown in FIG. 2, as the current flowing through the transistor 20 increases from zero, the emitter-base voltage VEB increases quite dramatically until the “knee” 22 of the curve is reached. Thereafter as the current increases further, the emitter-base voltage VEB is fairly constant despite large changes in the magnitude of the current. The current source 14 maintains the reference current IR at a magnitude that is greater than the magnitude of the current at the knee 22. As a result, the diode reference voltage VEB is maintained at a relatively constant value despite slight fluctuations in the magnitude of the reference current IR. A typical value for the diode reference voltage VBG is 0.65 volts, and the diode voltage generator 10 is able to maintain that voltage to within a few millivolts.
Diode voltage generators are often exposed to environments in which the temperature can vary widely, and yet it is important to maintain the reference voltage constant despite these temperature variations. Unfortunately, although the diode reference voltage VEB is substantially insensitive to small variations in the magnitude of the reference current IR, the voltage VEB is not insensitive to variations in the temperature of the transistor 20. In particular, the magnitude of the diode reference voltage VEB varies with temperature at about −2 mV/° C., as shown in the graph of FIG. 3. Circuits have therefore been developed to temperature compensate the diode reference voltage VEB shown in FIG. 1. An example of a temperature compensated diode voltage generator 30 is shown in FIG. 4. The reference voltage generator 30 uses a summing amplifier 34 to sum VEB from the PN junction diode of FIG. 1 with a thermal reference voltage VT generated by a thermal voltage source 36 to produce a reference voltage VR at its output. The thermal reference voltage VT is generated from a voltage generated by the thermal voltage source 36 after being boosted by a factor of K using an amplifier 38 having a gain of K.
The thermal reference voltage VT is 0.026V varying with temperature at 0.085 mV/° C. After this value is adjusted by a constant K to make the thermal voltage equal to 0.6 V, a temperature sensitivity of 1.96 mV/° C. is obtained (i.e., K=0.06 V/0.026 V). The sum of the 1.96 mV/° C. thermal sensitivity of the thermal reference voltage VT and the −2 mV/° C. thermal sensitivity of the diode forward voltage VEB results in a terminal sensitivity of the reference voltage VR of only about −0.04 mV/° C., which is substantially insensitive to temperature variations. A graph of the reference voltage VR, i.e., the sum of the thermal reference voltage VT and the diode forward voltage VEB, is shown in FIG. 6.
Although the temperature compensated diode voltage generator 30 can provide a precise reference voltage that is substantially insensitive to temperature, it is not without its limitations. In particular, as is apparent from FIG. 6, the diode voltage generator 30 generates a reference voltage VR of about 1.25 volts, which inherently requires a supply voltage of at least 1.25 volts. However, electronic devices are increasingly being powered by supply voltages of less than 1.25 volts, thus making the temperature compensated diode voltage generator 30 unsuitable for use in such devices. As a result, there is no relatively simple and inexpensive means to provide a precise, temperature compensated reference voltage in low voltage devices.
There is therefore a need for a method and system for generating a precise reference voltage that is substantially insensitive to temperature and that can be powered by a relatively low supply voltage.